add --color-scheme flag for persistent dark/light mode (#528)

Fixes #519. Playwright defaults `colorScheme` to `light` on all new contexts, overriding the browser/OS dark mode setting. This is especially disruptive in CDP mode, where every reconnection resets the scheme. The `set media dark` command also didn't persist its choice to new tabs or pages.

- Add `--color-scheme <dark|light|no-preference>` flag, config key (`colorScheme`), and env var (`AGENT_BROWSER_COLOR_SCHEME`)
- Store the preference in `BrowserManager` and automatically apply it to all new contexts (via Playwright's context option) and all new pages (via `page.emulateMedia` in `setupPageTracking`)
- `set media dark/light` now also persists its choice for subsequent pages and tabs

skills/agent-browser/SKILL.md

@@ -182,6 +182,19 @@ agent-browser --auto-connect snapshot
 agent-browser --cdp 9222 snapshot
 ```
 
+### Color Scheme (Dark Mode)
+
+```bash
+# Persistent dark mode via flag (applies to all pages and new tabs)
+agent-browser --color-scheme dark open https://example.com
+
+# Or via environment variable
+AGENT_BROWSER_COLOR_SCHEME=dark agent-browser open https://example.com
+
+# Or set during session (persists for subsequent commands)
+agent-browser set media dark
+```
+
 ### Visual Browser (Debugging)
 
 ```bash